The invention relates to gas lasers. More particularly the invention relates to holding and extraction devices for the optical elements of gas lasers.
Lasers have recently been applied to a large variety of technical areas, such as optical measurement techniques, material processing, medicine, etc.
Due to the special chemical, ablative, spectroscopic or diffractive properties of UV light, there is a big demand for lasers that generate laser beams having a short wavelength in the UV range.
Excimer lasers, such as the ones disclosed in U.S. Pat. Nos. 5,771,258 and 5,438,587, serve well as a laser for generating coherent, high intensity pulsed beams of light in the UV wavelength range.
The excimer lasers described in U.S. Pat. Nos. 5,771,258 and 5,438,587, are pulsed lasers. Pulsing is required in excimer lasers to allow sufficient time between pulses to replace the laser gas within the discharge region with fresh gas and allow the gas used for generating the previous pulse to recover before being used again for another gas discharge. In the discharge region (i.e., discharge gap), which in an excimer laser is typically defined between an elongated high voltage electrode and an elongated ground electrode which are spaced apart from each other, a pulsed high voltage occurs, thereby initializing emissions of photons which form the laser beam.
The laser beam is emitted along the extended ground electrode in a longitudinal direction of the laser tube. To achieve the desired amplification by stimulated emission of radiation, a resonator comprising a reflecting and a partially reflecting optical element disposed at opposite ends of the discharge gap is required. The laser beam leaves the tube through the latter.
If the reflective optical elements are provided outside the gas laser tube, a fully transparent window is provided in alignment with the discharge gap at each end of the tube to seal the tube, as can be seen in U.S. Pat. No. 5,438,587, for example. A mirror or other reflective optical element is then provided in axial alignment with one of the windows and its reflective side facing the window. A partially transparent, partially reflective mirror is positioned outside the tube so that it is aligned with and facing the other window. As a result, the faces of the two reflective optical elements are opposing one another and define a laser light resonator.
If the reflective optical elements are used to seal the tube, the mirror and the partially transparent, partially reflective mirror are integrated into the end walls of the tube at opposite ends of the discharge gap. As a result, no extra windows are required. For lasers emitting light in the ultraviolet range of the electromagnetic spectrum, extra windows have the disadvantage of significantly reducing the efficiency and increasing the operating costs, as the special window materials employed are expensive and deteriorate with use and time and need to be occasionally changed. In addition, the transparent windows closing the tube form extra optical elements resulting in extra losses and reflections on the surfaces. The latter can be removed by inclining the window at Brewster""s angle as taught by U.S. Pat. No. 4,746,201, but invariably the laser output is reduced. Deterioration of the optical elements also cannot be entirely avoided, reducing output and giving rise to the need to replace the rather expensive optical elements after a certain time.
Within the laser""s resonator, the laser light resonates between the fully reflective mirror and the partially transmissive, partially reflective mirror to amplify the laser effect. In addition, a portion of the resonating light is emitted through the partially transmissive, partially reflective mirror at the target.
The reflective optical elements that form the resonator must be precisely positioned relative to one another to ensure optimal laser light output power, laser efficiency, and the quality of the laser beam. This is especially true with respect to the angular alignment of the reflective optical elements, not only with respect to each other, but also with respect to the laser tube. However, maintaining the appropriate angular alignment of the reflective optical elements is difficult in view of changes in the operating conditions, such as pressure or temperature of the gas and the temperature of the tube, the optical elements, and their supporting units. In addition, mechanical vibrations or shock to the laser may also affect the angular alignment of the reflective optical elements forming the laser resonator.
As is known in the art, the reflective optical elements forming the resonator may be provided inside or outside the laser tube. Regardless of whether the reflective optical elements are positioned inside or outside the laser tube, however, an optical element of some sort must be mounted to the laser tube to seal the laser tube while allowing laser light to be transmitted out of the laser tube. Thus, when the reflective optical elements are used to seal the tube, they are integrated into the end walls of the tube at opposite ends of the discharge gap and thus are used to seal the tube. On the other hand, if the reflective optical elements forming the resonator are provided outside the laser tube, then fully transparent windows are provided at opposite ends of the tube to seal the tube. It is known that these optical elements, both reflective and transmissive, may be secured to the laser tube by means of a flange fixed by screws. This known securing mechanism, however, has many disadvantages. These disadvantages include:
1. The central portion of the optical element is blackened on its internal side, i.e. on the laser side of the window. This results in the central portion of the optical element quickly deteriorating.
2. When the optical element is detached from the laser, for cleaning for example, the optical element frequently falls out of the securing device in which the optical element is inserted during normal operation and is thereby permanently damaged.
3. Further, because the optical element is typically fixed with screws to the end of the laser tube, it has not been possible or practical to turn the window in the securing mechanism. However, a securing mechanism that would allow the optical element to be rotated about its central axis would be desirable, for instance to allow the laser beam to pass through a portion of the optical element that is not blackened.
4. In smaller gas lasers it has been especially difficult to extract the optical element from the end of the laser tube, as there is very little space for obtaining access to the edge of the optical element without damaging it. This problem is further exacerbated by the fact that the optical element frequently adheres to an O-ring provided on the end wall of the laser tube, and which provides a gas-tight seal between the end wall of the tube and the optical element.
The present invention may be used in conjunction with the inventions described in the patent applications identified below and which are being filed simultaneously with the present application:
All of the foregoing applications are incorporated by reference as if fully set forth herein.
An object according to a first aspect of the invention is to provide an optical element holding and extraction device for a gas laser that permits improved maintenance characteristics of the optical element and thus is useful for extending the life of the optical element.
In order to achieve the first object, an optical element holding and extraction device is provided. The device includes an optical element, an optical element holder having a tubular gripping portion and a tubular extraction portion connected at one end to the tubular gripping portion, and a retainer that is slideably carried on the tubular extraction portion. The diameter of the tubular extraction portion is less than the tubular gripping portion. In addition, the tubular gripping portion grips or holds the peripheral edge of the optical element. The device according to the present object of the invention preferably further comprises a mounting structure comprising an optical element receiving surface. The retainer is removeably engaged with the mounting structure and secures the optical element against the optical element receiving surface.
Because the optical element is held in the gripping portion of the optical element holder and the optical element holder and optical element are removeably secured by the retainer to the mounting structure, the maintainability of the window is improved. Indeed, with the device according to the present invention it is now possible to readily and safely detach the holder and optical element from the mounting structure. Therefore, the optical element does not need to be pried from the mounting structure, which is especially difficult in smaller gas lasers, as described above. Instead, the optical element may be detached from the holding and extracting device after the holder and optical element are removed, together with the retainer, from the mounting structure. Furthermore, the optical element can be removed from the holder in a location where there is more working space a available. Thus, the replacement and maintenance of the optical element becomes much more comfortable. Furthermore, the optical element does not tend to fall out of the mounting structure anymore, because it is received and held by the optical element holding and extraction device.
Pursuant to a second object of the invention, it is an object to provide a gas laser having an optical element, wherein the maintainability of the optical element is improved.
To achieve the second object according to the invention a gas laser is provided that comprises a tube having a first end wall at one end and a second end wall at the other end. The tube defines a cavity for containing a laser gas therein, and the first end wall includes a port. An optical axis extends longitudinally through the tube and passes through the port. The laser further comprises a mounting structure mounted on the exterior wall of the first end wall of the tube. The mounting structure includes an optical element receiving surface and an aperture extending through the receiving surface. The aperture is disposed transverse to the optical axis and is aligned with the port and the optical axis so that the optical axis passes through the aperture. An optical element and an optical element holder are also provided. The holder comprises a tubular gripping portion and a tubular extraction portion connected at one end to the tubular gripping portion and has a diameter less than that of the tubular gripping portion. The tubular gripping portion grips or holds the peripheral edge of the optical element so that the optical element is secured in the optical element holder. A retainer is slideably and rotateably carried on the tubular extraction portion of the holder. The retainer is also removeably engaged with the mounting structure and secures the optical element against the optical element receiving surface to form a gas tight seal therebetween. The optical element is disposed transverse to the optical axis and the optical axis impinges on the optical element.
The gas laser according to the second aspect of the invention has the same advantages as the optical element holding and extraction device according to the first aspect of the invention. Furthermore, by employing the optical element holding and extraction device according to the present invention in a laser, damage to the laser itself may be prevented. As a result, it is now much easier to detach optical elements from lasers, thereby minimizing the potential of mechanically damaging the optical element or the laser when trying to detach the optical element from the laser tube.
The further features or embodiments described below are also suitable for the stand-alone optical element holding and extraction device according to the present invention or gas lasers employing the device.
For example, the optical holding and extraction device is preferably designed so that the retainer may be loosened without completely disengaging it from the mounting structure, and once the retainer is loosened the holder is rotateable within the retainer about a common axis. In addition, preferably when the holder is rotated the optical element is rotated as well.
With this embodiment, it is now possible to rotate the optical element while the optical element is still secured in the mounting structure without ventilating the laser system. This is especially advantageous when the laser light eccentrically impinges on the optical element because the lifetime of the optical element can be extended significantly. The lifetime of the optical element may be extended with this embodiment because it is now possible to occasionally rotate the optical element when the point where the laser beam impinges becomes too blackened. In other words, the optical element may be rotated so that the laser beam impinges on a fresh or clean portion of the optical element, thereby restoring the laser""s efficiency. Furthermore, the rotation can be carried out a number of times until the optical element has been rotated by about 360xc2x0, thus multiplying the window""s lifetime.
Other objects, features and advantages of the invention will become apparent to those skilled in the art from the following description of the preferred embodiment taken together with the drawings.